Female infidelity is the primary context for sperm competition in socially monogamous birds. Female infidelity places her partner at risk of investing in offspring to whom he is genetically unrelated and provided selection pressure on ancestral males to detect the infidelity and to inflict costs such as physical assault on unfaithful females. The costs inflicted by male birds on unfaithful partners reveal a psychology designed to motivate behaviors that prevent future infidelity or that "correct" recent infidelity by, for example, motivating immediate copulation. The social monogamy of birds--and the female infidelity that is the primary context for sperm competition--parallels the mating system of humans. Human female infidelity is associated with her personality, her partner's personality, discrepancy in mate value between her and her partner, and her partner's occupational setbacks. The costs that men inflict on unfaithful partners include physical abuse and forced copulation. Human males, like bird males, appear to be designed to solve the adaptive problems of sperm competition. Studying couples in committed relationships, evolutionary biologists found a large positive correlation between the risk of sperm competition-- operationalized as the time apart since last copulation--and the number of sperm a male inseminates into his partner at the couple's next copulation. Arguing that there must be psychological mechanisms guiding these physiological adjustments, evolutionary psychologists hypothesized and found positive correlations between the risk of sperm competition and, for example, men's sexual interest in their partner. These relationships were not hypothesized or found for women, because ancestral women did not face the adaptive problem of their partner being inseminated by a rival and, therefore, will not have evolved mechanisms designed to solve this problem. The mechanisms that may have evolved to solve the adaptive problems of sperm competition are proposed to be universally present in male psychology, yet the samples for the pervious work comprised only young college students. In the proposed project, 45 married couples varying in age, ethnicity, and education will be recruited via newspaper advertisements and visits to the Department of Motor Vehicles to participate in exchange for 100 dollars. The previous work secured a single assessment of psychological reactions to time apart since last sexual intercourse, precluding causal analyses of the relationships between risk of sperm competition and reactions to risk of sperm competition. In the proposed project, couples will participate in a one-month daily study that will allow such causal analyses. The previous work relied on self-reported sexual beliefs, behaviors, and desires. Two variables in particular could be profitably assessed by self-report and partner-report: Number of hours since last sexual intercourse and number of hours spent apart since last sexual intercourse. The proposed project will secure daily partner-reports and self-reports of these and other key variables. Finally, the proposed project will extend previous work by testing hypothesized relationships between time apart since last sexual intercourse and men's partner-directed physical abuse, sexual coercion, and other tactics of mate retention.